Determine the percent aspirin in an aspirin tablet and to compare this with the amount specified on the label.

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Titrimetric Analysis of Aspirin

Name:                 Andrew Holmes

Student         Number: 2103086

Course:         BSc Forensic Science

Year:                 1

Unit:                 Introduction to Forensic Chemistry

Date:                 18/03/02


The purpose of the experiment was to determine the percentage of acetyl salicylic acid in an aspirin tablet and to compare this with the amount specified on the label.  The average weight of one tablet before titration was found to be 328 mg.  The amount of aspirin calculated from the results obtained from titrimetric analysis to be 317 mg. The stated dose of the tablets was 300 mg The percent purity of an aspirin tablet to the amount specified on the label was then calculated to be 96.47 % pure.

Aims and Objectives


The objectives of this experiment are to determine the percent aspirin in an aspirin tablet and to compare this with the amount specified on the label. This is carried out using an acid – base titration.


The aims of the experiment were as follows:

  • Calculate the weight of one aspirin tablet.
  • Calculate the molarity of diluted and stock NaOH solutions.
  • Calculate the no. Of moles used in hydrolysis.
  • To determine the percent aspirin in an aspirin tablet and compare it with the amount specified on the label.
  • Calculate the percent purity of aspirin in one aspirin tablet to the specified amount on the label.


A titration is the basis of titrimetiric analysis. It is in which a solution containing a known concentration of a reagent is added to a solution containing an unknown concentration of a substance (In this case the sodium hydroxide remaining after hydrolysing acetyl salicylic acid) until the reaction between the reagent and the substance is complete. The solution of known concentration is called a standard solution and is the titrant (In this case hydrochloric acid). The volume of titrant added is measured and since its molarity is known the exact molarity of the reagent can be calculated

The titrant is added until the reaction is just complete at a point called the end point of the titration. An indicator is added to show when the end point has been reached; an indicator is some substance which changes colour at the end point and therefore show when the titration should be stopped.  

The indicator being used in this experiment is phenolphthalein. Phenolphthalein is a weak acid, which is ionised in solution:

The acid itself is colourless but the anion P- is red. If the indicator is placed in an acidic solution where the hydrogen ion concentration is comparatively high, the dissociation of the indicator is suppressed. The unionised acid is colourless and therefore the solution is also colourless. IN alkaline solutions the hydroxyl ions remove the hydrogen ions produced in the above reaction, hence the concentration of P- ions is increased and the solution is given a pink colour.

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 The change from colourless to pink starts at pH = 8.2 and is complete by pH = 9.8. It is ideal for this experiment as its stoichiometeric point is near 9 which is ideal for a titration using a weak acid (acetyl salicylic acid) and a strong base (sodium hydroxide)

Aspirin is an analgesic drug that can be harmful if taken in excess. The main constituent is 2-ethnoylxybenzoic acid (acetyl salicylic acid.) The amount of aspirin in a sample can be quantified using its acidic properties in an acid-base reaction.

A known mass of aspirin is hydrolysed ...

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